Project Summary The cell cycle control system in the protozoan parasite Trypanosoma brucei possesses many unusual features, among which mitosis and cytokinesis represent the most unusual aspects to be explored in more detail. Mitosis in T. brucei likely involves novel mechanisms because T. brucei does not possess centrosomes/spindle pole bodies as the microtubule organizing centers for mitotic spindle assembly and there are insufficient numbers of kinetochores per chromosomes (Ogbadoyi et al., 2000). This challenges the wellrecognized spindle microtubule-kinetochore mechanism for chromosome segregation. Similarly, cytokinesis in T. brucei also differs from its human host, but little is known about its regulation at the molecular level. Unlike animal cells that divide along the shortest cellular axis through constriction of an actomyosin contractile ring (Oliferenko et al., 2009), cytokinesis in T. brucei is initiated from the anterior tip of the new flagellum attachment zone (FAZ) and proceeds along the longitudinal axis toward the posterior end without forming an actomyosin ring (Kohl et al., 1999;Li et al., 2008a). Cytokinesis initiation in animals involves Aurora B kinaseand Polo-like kinase (PLK)-mediated phosphorylation of the centralspindlin complex at the central spindle and subsequent activation of the RhoA GTPase at the furrow for further activation of actin and myosin to form the actomyosin ring (Carmena, 2008). Both Aurora B kinase and PLK are required for cytokinesis in T. brucei, but the absence of the centralspindlin complex and the actomyosin ring indicates that the mechanism of cytokinesis in trypanosome is distinct from its human host. The current proposal is built upon the identification of the chromosomal passenger complex (CPC) and TbPLK as essential regulators of mitosis and cytokinesis in T. brucei, and aims to address the following questions. How does the CPC regulate mitosis? This is still poorly understood, mainly because the downstream factors are not identified. Further, after fulfilling its function in mitosis, the CPC moves from the nucleus to the anterior tip of the new FAZ to initiate cytokinesis, but how this migration is regulated remains elusive. Finally, since both the CPC and TbPLK are enriched at the anterior tip of the new FAZ and are required for cytokinesis, we ask whether the CPC cooperates with TbPLK to regulate cytokinesis initiation by forming a complex or regulating some common downstream factors. Our central hypothesis is that the CPC regulates some novel proteins in the nucleus to promote mitosis and subsequently migrates to the anterior tip of the new FAZ where it cooperates with TbPLK and other novel proteins to initiate cytokinesis. Through genetic and biochemical means, our overall goal in this proposal is to understand the mechanistic role of the CPC in mitosis, mitosis-cytokinesis coordination through regulation of CPC dynamics, and cytokinesis initiation mediated by cooperative action of the CPC and TbPLK. The long-term goal of my laboratory is to delineate the regulatory networks that control mitosis and cytokinesis in T. brucei, which will facilitate our fundamental understanding of the molecular basis of mitosis, mitosis-cytokinesis coordination, and cytokinesis that is totally different from the commonly recognized cell division through the constriction of an actomyosin contractile ring. The outcome from these studies would not only have important biological significance, but also could provide novel targets for anti-trypanosomiasis chemotherapy.